1. Field of the Invention
This invention relates to an optical apparatus to control an inner focus type lens system.
2. Description of the Related Art
It is known that an inner focus type lens system includes a focusing-compensator lens which is arranged in rear of a variator lens on its optical axis to serve both as a compensator lens and a focusing lens. Therefore, the position of the focusing-compensator lens at which an in-focus state is obtainable on an image pickup plane varies with a distance to a picture-taken object even if the focal length of the optical system is kept constant.
When the object distance is changed at each of different focal lengths, the position of the focusing-compensator lens for obtaining an in-focus state on the image pickup plane varies as indicated by points continuously plotted in FIG. 2. A zooming operation can be carried out without any blur by selecting a specific locus from among a plurality of loci shown in FIG. 2 according to the object distance and by moving, during zooming, the focusing-compensator lens according to the specific locus.
Meanwhile, in the case of a front-lens focus type lens system, a compensator lens is arranged independently of a variator lens. Further, the variator lens and the compensator lens are connected to each other by a mechanical cam ring. Therefore, if, for example, a manual zooming operation ring is provided on the cam ring for manually varying the focal length, the cam ring follows the operation ring to enable the variator and compensator lenses to move along the cam groove of the cam ring, no matter how fast the operation ring is moved. Therefore, no blur will be caused by the movement of the operation ring as long as the focusing lens is in an in-focus position.
With respect to the control over the inner focus type lens system which has the characteristic mentioned above, it is generally practiced to have information on the plurality of loci shown in FIG. 2 stored in a suitable form in a lens control microcomputer, to select one of the loci according to the position of the focusing-compensator lens and that of the variator lens, and to perform zooming while tracing the locus selected.
According to the conventional practice, however, a moving speed of the focusing-compensator lens and a point to which the focusing-compensator lens is to be moved are determined according to the position of the variator lens. Therefore, the conventional practice has the following drawbacks:
(1) If the position of the variator lens is not accurately detected within a given allowable error range, it is hardly possible to correctly read out the locus data corresponding to the actual position of the variator lens. The inaccurate detection of the position of the variator lens degrades the accuracy of focusing to be carried on during the process of zooming. Therefore, it is necessary to use a highly accurate detector for the detection of the position of the variator lens. However, the use of an accurate detector results in a cost increase.
As apparent from FIG. 2, the locus data gradually approaches to a shape perpendicular to the abscissa axis near a telephoto end position of the variator lens. It is known that this tendency becomes more salient accordingly as the magnifying rate of the variator lens is higher. In order to accurately store the locus data for positions near the telephoto end, therefore, the position detecting accuracy for the variator lens must be increased for a greater magnifying power of the lens system.
(2) Since the focusing-compensator lens must be smoothly moved according to the movement of the variator lens, the moving speed of the variator lens must be kept constant. In cases where a DC motor is used for driving the variator lens like the conventional arrangement, it is necessary for this purpose to apply servo-control to the driving action of the DC motor or to finely adjust the movement of the focusing-compensator lens by relatively accurately detecting the fluctuations in the moving speed of the variator lens during zooming.
The necessity of the servo-control and the detection of fluctuations in the moving speed not only imposes restrictions on design but also directly causes increases in the number of parts and in the load of microcomputer programs.